Moment induced total arthroplasty prosthetic

ABSTRACT

A prosthetic total knee replacement system comprises a distal femoral implant component, a tibial tray implant component and a mobile bearing tibial tray insert. The mobile bearing tibial tray insert is rotatably mountable on the tibial tray for articulation with the distal femoral implant component. The mobile bearing tibial tray insert rotates with respect to the tibial tray around a substantially vertical first rotational axis offset laterally from the medial-lateral centerline of the mobile bearing tibial tray insert and the distal femoral implant component rotates with respect to the tibial tray insert about a substantially vertical second rotational axis offset medially from the first rotational axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/203,987, now U.S. Pat. No. 8,636,807, issued on Jan. 28, 2014, whichis a continuation that claims priority under 35 U.S.C. §120 to PatentCooperation Treaty Application Serial No. PCT/US2006/010431 filed onMar. 21, 2006, entitled “MOMENT INDUCED TOTAL ARTHROPLASTY PROSTHETIC,”the disclosures and amended disclosure of which are hereby incorporatedby reference in their entireties.

RELATED ART

1. Field of the Invention

The exemplary embodiments disclosed herein are directed to implantedprosthetic devices and, more specifically, to implanted prostheticjoints that simulate the natural biomechanics of native mammalianjoints. Further exemplary embodiments also encompass methods of inducingmoments within an implanted prosthetic joint, as well as implantedprosthetic devices that incorporate induced moments.

2. Brief Discussion of Related Art

Previous research studies have documented that the amount of rotationoccurring in the knee, during walking and high flexion activities,varies greatly between a natural knee (no prior surgeries and no historyof clinical abnormalities) and a surgically revised knee such as thosehaving undergone total knee arthroplasty (TKA). The axial femorotibialrotational pattern of the femur relative to the tibia during flexion ofa natural knee induces tibia internal rotation relative to the femur,and tibia external rotation relative to the femur as the knee extends.This internal rotational mechanism results in part from the length andtension within the cruciate and collateral ligaments and anatomicvariances in dimensions of the medial and lateral femoral and tibialcondyles.

The exact axial femorotibial rotational pattern after TKA is less clear,but has been shown to vary considerably. Many have hypothesized thatdecreases in axial rotation after TKA may be related to removal oralteration of the cruciate ligaments and/or failure to exactly duplicategeometry of the medial and lateral femoral and tibial condyles, althoughmost hypothesize that this reduced motion derives from the inability toreproduce correct condylar geometries.

A multicenter, in vivo, weight-bearing kinematic analysis hasdemonstrated similar average axial rotational values in fixed versusmobile bearing TKA prosthetics. Controversy exists, however, as to theexact site of axial rotation (superior vs. inferior aspect of the tibialinsert) in mobile bearing TKA prosthetics. Many fixed and mobile bearingTKA prosthetics have demonstrated significantly reduced axial rotation,while others have documented that TKA patients often experience areverse rotation pattern, where the tibia externally rotates about thefemur with increasing knee flexion.

INTRODUCTION TO THE INVENTION

The exemplary embodiments disclosed herein are directed to implantableprosthetic devices simulating the natural biomechanics of nativemammalian joints. These exemplary embodiments induce moments within animplantable prosthetic joint to impart rotational movement between jointcomponents. The exemplary embodiments also encompass the jointcomponents themselves that induce moments, as well as methods forinducing moments to impart rotation between complimentary prostheticcomponents within an implantable joint.

In one aspect, the present invention provides a prosthetic knee implantsystem comprising a distal femoral component, a tibial tray and a mobilebearing tibial tray insert. The mobile bearing tibial tray insert isrotatably mountable on the tibial tray for articulation with the distalfemoral implant component. The distal femoral implant component has amedial condyle and a lateral condyle and the mobile bearing tibial trayinsert has a medial condyle receiver spaced apart from a lateral condylereceiver. The mobile bearing tibial tray insert rotates with respect tothe tibial tray around a substantially vertical first rotational axisoffset laterally from the medial-lateral centerline of the mobilebearing tibial tray insert and the distal femoral implant componentrotates with respect to the tibial tray insert about a substantiallyvertical second rotational axis offset medially from the firstrotational axis.

In a more detailed embodiment, the first rotational axis is offsetlaterally from a medial-lateral centerline of the mobile bearing tibialtray insert.

In another more detailed embodiment, the first rotational axis is offsetanteriorly from an anterior-posterior centerline of the mobile bearingtibial tray insert. Alternatively, the first rotational axis may beoffset posteriorly from an anterior-posterior centerline of the mobilebearing tibial tray insert.

In another more detailed embodiment, the lateral condyle receiverincludes at least one of a convex shape or a sequentially sloped fromanterior to posterior shape and the medial condyle receiver includes aconcave shape.

In another more detailed embodiment, the mobile bearing tibial trayinsert includes a recess adapted to receive a projection from the tibialtray allowing the mobile bearing tibial tray insert to rotate withrespect to the tibial tray about the first rotational axis.Alternatively, the mobile bearing tibial tray insert may include aprojection adapted to be received by a recess within the tibial trayallowing the mobile bearing tibial tray insert to rotate with respect tothe tibial tray about the first rotational axis

In another more detailed embodiment, there is translational laxitybetween the medial condyle receiver and the medial condyle duringflexion of the knee.

In another more detailed embodiment, the medial condyle receiver and themedial condyle are shaped to have up to 12.5 mm of translational laxityat greater than 20 degrees of knee flexion.

In another more detailed embodiment, the medial condyle is larger thanthe lateral condyle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a free body diagram of a cylinder, B, contacting a platform,A;

FIG. 2 is a diagram representing the curvature of a femoral condyle as alinear segment and showing the amount of the linear segmentcorrespondingly contacted during flexion of the knee joint;

FIG. 3 is an overhead view of a prior art posterior stabilized mobilebearing total knee arthroplasty prosthetic insert having thepolyethylene post aligned over the center of rotation;

FIG. 4 is an overhead view of a prior art fixed bearing posteriorstabilized total knee arthroplasty prosthetic insert having thepolyethylene post located along the medial-lateral midline of thepolyethylene insert;

FIG. 5 is an overhead view of a prior art posterior cruciate retainingtotal knee arthroplasty prosthetic insert;

FIG. 6 is a rear, profile view of a first exemplary moment inducedmobile bearing posterior stabilized total knee arthroplasty prostheticin accordance with the present invention;

FIG. 7 is an overhead view of a first exemplary moment induced mobilebearing posterior stabilized total knee arthroplasty prosthetic insertin accordance with the present invention;

FIG. 8 is a plot of polyethylene rotation versus time for a prior artpolyethylene tibial insert having the post and the rotation point in thecenter of the polyethylene, and the new moment induced posteriorstabilized mobile bearing total knee arthroplasty prosthetic insertshowing substantially greater rotation;

FIG. 9 is a rear profile view of a first exemplary moment induced fixedbearing posterior stabilized total knee arthroplasty prosthetic insertin accordance with the present invention;

FIG. 10 is an overhead view of a first exemplary moment induced fixedbearing posterior stabilized total knee arthroplasty prosthetic insertin accordance with the present invention;

FIG. 11 are medial and lateral profile views of exemplary changes inmedial condyle conformity and increased lateral condyle posterior slopefor the exemplary prosthetic inserts of the present invention;

FIG. 12( a) is a pictorial representation of a prior art lateral femoralcondyle PCR TKA prosthetic having a rounded convex shape;

FIG. 12( b) and FIG. 12( c) are pictorial representations of anexemplary MITKA PCR prosthetic having femoral radii for the lateralcondyle being flatter, in contact with either a convex or slopeddownward in the posterior direction tibial component.

FIG. 13 is an overhead view of a first exemplary moment induced mobilebearing posterior stabilized total knee arthroplasty prosthetic insertin accordance with the present invention;

FIGS. 14( a) & (b) are pictorial profile representations of an exemplaryMITKA posterior cruciate retaining TKA having increased conformitybetween the medial condyle and the medial receiver of the tibial insertat varying degrees of knee flexion;

FIGS. 14( c) & (d) are pictorial profile representations of an exemplaryMITKA posterior cruciate retaining TKA having an increased slopedbetween the lateral condyle and the tibial insert at varying degrees ofknee flexion;

FIGS. 14( e) & (f) are pictorial profile representations of an exemplaryMITKA posterior cruciate retaining TKA with a lateral convex shape alongwhich the lateral condyle rides at varying degrees of knee flexion;

FIG. 15 is a 3-D overhead, perspective view of a prior art TKAprosthetic cam/post mechanism with flat leading surfaces;

FIG. 16 is an elevated perspective view of an exemplary MITKA posteriorstabilized prosthetic insert; and

FIG. 17 is an elevated perspective view of an exemplary MITKA posteriorstabilized prosthetic device.

DETAILED DESCRIPTION

The exemplary embodiments described and illustrated below encompassmethods of inducing moments within an implanted prosthetic joint, aswell as implantable prosthetic joints and components thereof inducingmoments. Of course, it will be apparent to those of ordinary skill inthe art that the embodiments discussed below are exemplary in nature andmay be reconfigured without departing from the scope and spirit of theinvention. However, for clarity and precision, the exemplary embodimentsas discussed below may include optional steps, methods, and featuresthat one of ordinary skill should recognize as not being a requisite tofall within the scope of the claims.

Basic principals governing the laws of mechanics are taken from Newton'sLaws: (A) every object in a state of uniform motion tends to remain inthat state of motion unless an external force is applied to it; (B) therelationship between an object's mass m, its acceleration a, and theapplied force F, is F=ma; and, (C) for every action there is an equaland opposite reaction. The above laws of mechanics pertain to externalforces applied to a system, however, when an in-plane force is appliedto an object that has the ability to move, and if the applied force isgreater than the resistive force (gravity, friction, etc.), the objectwill begin to move. Throughout knee flexion, whether during gait or intodeep flexion, the cruciate ligaments of a natural knee force the tibiato internally rotate, levering the femur with respect to the tibia. Ithas been documented that the absence of the cruciate ligaments leads toa decrease in axial rotation.

For total knee arthroplasty (TKA) prosthetics, three primary forces maybe exerted thereupon: (1) applied forces, which are produced by musclespassing across the knee joint; (2) bearing surface contact forcesoccurring between the femur and the tibia at the contact points andbetween the femur and the patella at the contact points; and, (3)constraint forces produced by ligaments resisting the active forces.However, the incidence and magnitude patterns of normal axial rotationof a knee prosthesis is governed by, and can only be induced to rotateby introducing moment arms with respect to active forces to causerotation. In an exemplary system, a vector V has a distance D, with aline of action passing through a starting point P of the vector V. Themoment M of the vector V about point P is characterized by Equation #1:M=R×V; where R is the position vector from point P to a second pointalong the vector V.

Before a moment analysis can be conducted for any TKA prosthetic, anunderstanding of the forces acting on the knee, both magnitude anddirection, should be clearly determined and understood. The mosteffective method for deriving muscle, bearing surface and ligamentforces, simultaneously, is through mathematical modeling techniques. Ithas been demonstrated that, with a proper understanding of kneemechanics, it is possible to derive equations to determine in vivoforces. Although it is important to know the magnitude of the forcesapplied at the knee, it is equally important to determine the directionof those applied forces. Proper direction of contact forces acting atthe femorotibial and patellofemoral interfaces will ensure propersummation of the moments about a chosen point. Therefore, it isimportant to determine the direction of the velocity of the point on thefemur, point FT, in contact with the tibia, which will allow for thedetermination of the direction of the bearing surface contact forceoccurring between points FT and TF, which is the point on the tibia incontact with the femur.

In a natural knee, like any mechanical system that has any two objectsin contact, three possible conditions could occur, which lead to vastlydifferent conditions at the contact point between the two objects. Thesethree conditions are: (1) pure rolling; (2) pure slipping; and, (3) acombination of rolling and slipping.

Referencing FIG. 1, an exemplary free-body diagram 100 includes a roundcylinder (Body A) 102 with radius R moving with respect to a generallyplanar platform (Body B) 104 that is fixed in the Newtonian referenceframe. In this simple example, two reference frames are defined for eachobject 102, 104, where the “A2>” and “B2>” directions are opposite ofgravity. The point of contact between the objects 102, 104 is mutuallydefined by two points: point AB on the platform 104, and point BA on thecylinder 102. Three other points P1, P2, P3 are equidistantly spacedaround the circumference of the cylinder 102, with the longitudinalcenter being identified by point BO. Point P1 is spaced a distance R inthe A1> direction and a distance R in the A2> direction from point BA.Point P2 is spaced from point BA a distances R in the −A1> direction andR in the A2> direction. Point P3 is spaced a distance 2 R in the A2>direction, from point BA.

Under pure rolling conditions, we can assume the velocity vectorV_BO_A>=A1>, where: the radius R=1; and, the angular velocity of thecylinder ω, relative to the reference frame for the platform around theA3> axis, is equal to −A3>. One can then determine the velocity forpoints P1, P2, P3 and BO, which are determined using Equations #2-#5:V _(—) P1_(—) A>=V _(—) BO _(—) A>+ω _(—) B _(—) A>×P _(—) BO _(—) P1>V _(—) P1_(—) A>=A1>+−A3>×A1>V _(—) P1_(—) A>=A1>−A2>  Equation #2:V _(—) BA _(—) A>=V _(—) BO _(—) A>+ω _(—) B _(—) A>×P _(—) BO _(—) BA>V _(—) BA _(—) A>=A1>+−A3>×−A2>V _(—) BA _(—) A>=A1>−A1>=0>  Equation #3:V _(—) P2_(—) A>=A1>+A2>  Equation #4:V _(—) P3_(—) A>=2·A1>  Equation #5:Therefore, under pure rolling conditions, the velocity of point BA mustequal the velocity of point AB. Since the platform 104 is “fixed” andnot moving in the Newtonian reference frame, all points on the platformhave a velocity equal to zero. This simple analysis shows that thevelocity of point BA, on the cylinder 102, is equal to zero, under purerolling conditions.

Under pure slipping conditions, the velocities for this same system,shown in FIG. 1, are different for each point along the cylinder 102. Apractical way to describe pure slipping would be a car on ice. If thefriction coefficient were equal to zero, the tires would spin, but thecar would remain stationary. Therefore, in the knee, under pure slippingthe V_BO_A>=0>, and similar to our example shown in FIG. 1, theRadius=1, ω=−A3>. Then the velocities for points BA, P1, P2, and P3 aredetermined using Equations #6-#9:V _(—) P1_(—) A>=V _(—) BO _(—) A>+ω _(—) B _(—) A>×P _(—) BO _(—) P1>V _(—) P1_(—) A>=0>+−A3>×A1>V _(—) P1_(—) A>=−A2>  Equation #6:V _(—) BA _(—) A>=V _(—) BO _(—) A>+ω _(—) B _(—) A>×P _(—) BO _(—) BA>V _(—) BA _(—) A>=0>+−A3>×−A2>V _(—) BA _(—) A>=−A1>  Equation #7:V_P2_A>=A2>  Equation #8:V_P3_A>=A1>  Equation #9:Therefore, under pure slipping, the velocity of point BA is equal to−A1>, and the direction of the velocity is opposite in direction toposterior femoral rollback of the femoral condyles in a knee. Althoughit has been assumed that the velocity vector of the contact pointbetween the femoral condyles and the tibial plateau would be in theposterior direction, under pure slipping, the correct direction of thevelocity vector is in the anterior direction during flexion and in theposterior direction during extension. Although pure rolling and pureslipping have been described, it can be assumed that, under in vivoconditions, “only” pure rolling or “only” pure slipping conditionscannot occur.

Referencing FIG. 2, a circumferential distance 200 of a prostheticfemoral condyle 202 can be represented by a flat line 204. Under purerolling conditions, the prosthetic femoral condyle 202 would follow theflat line 204 path, which is much greater in distance than ananterior/posterior dimension of a prosthetic tibial insert (not shown).Previous analyses have documented that the amount of anterior posteriormotion for a natural knee can range between 10 to 25 mm for the lateralcondyle and, for a TKA prosthetic, this motion could be 10 mm in theanterior direction or 15 mm posterior. Thus, in a TKA prosthesis, themost dominant motion occurring at the contact point between the femoralcondyle 202 and the tibial insert is slipping.

Referring again to FIG. 1, an analysis can be conducted to determine thevelocities on the cylinder 102 at the bearing surface interface BA, ABwhen both slipping and rolling are present. In this analysis, we canassume V_BO_A>=A1>, radius R=1, and the angular velocity of the cylinderω=−2A3>. Therefore, the velocities for points P1, P2, P3 and BA can bedetermined using Equations #10-#13:V _(—) P1_(—) A>=V _(—) BO _(—) A>+ω _(—) B _(—) A>×P _(—) BO _(—) P1>V _(—) P1_(—) A>=A1>+−2A3>×A1>V _(—) P1_(—) A>=A1>−2A2>  Equation #10:V _(—) BA _(—) A>=V _(—) BO _(—) A>+ω _(—) B _(—) A>×P _(—) BO _(—) BA>V _(—) BA _(—) A>=A1>+−2A3>×−A2>V _(—) BA _(—) A>=A1>−2A1>=−A1>  Equation #11:V _(—) P2_(—) A>=A1>+2A2>  Equation #12:V _(—) P3_(—) A>=A1>+2A2>=3A2>  Equation #13:

Under all three conditions (slipping, rolling, or a combination), animportant piece of information is the velocity of point BA. During purerolling, the velocity of point BA is equal to zero, but under pureslipping and a combination of rolling and slipping, in our examples,this velocity is not equal to zero. Under pure slipping the direction ofthe velocity BA is in the −A1> direction, opposite of the direction ofposterior femoral rollback of the femoral condyles. During a combinationof rolling and slipping, the direction of this velocity vector, V_BA_N>,in our example is in the −A1>, which is, again, in the oppositedirection of contact point BA on AB. The magnitude of V_BA_N> canchange, depending on the velocity of BO and the angular velocity of bodyB in the Newtonian reference frame, but the magnitude will always be inthe −A1> direction. Therefore, it is disadvantageous to design a totalknee arthroplasty prosthesis assuming that the forces at point BA on ABduring knee flexion are in the A1> (posterior direction) direction.Instead, one should design a total knee arthroplasty prosthesis with theforces being applied in the −A1> direction (anterior direction) duringknee flexion and in the A1> direction during knee extension, similar tothe direction of velocity vector acting at this point. Also, it shouldbe noted, that during flexion the velocity of the contact point BA isequal to zero, under pure rolling and is in the anterior direction(−A1>) under pure slipping. Therefore, during knee flexion, V_BA_N> isnot in the posterior direction.

At present, all known TKA prosthetics are designed for equaldistribution of forces at the contact points between the femoralcomponents and the tibial components. Therefore, these TKA prostheticsdo not incorporate moments to create axial rotation. During surgery, thegoal of the surgeon is to create equal tension gaps between the femoralcondyles and the tibial insert/plateau. If the amount of force actingbetween the medial condyle and the tibial insert is equal to the forceacting between the lateral condyle and the tibial insert, it could beexpected that the femoral components will not achieve axial rotationrelative to the tibial insert because the medial and lateral condyledistances from the center of the tibial insert are also the same. If twoforces act on a system and both forces are equal in magnitude and themoment arms to those forces, from a fixed point, are equal, then themoment of this system would be equal to zero.

Referencing FIG. 3, a typical posterior stabilized mobile bearing TKAprosthetic 300 accommodates five main contact forces: (1) the medialcondyle force 302 in the vertical direction (F^(N) _(M)); (2) the medialcondyle force 304 in the anterior/posterior direction (F^(T) _(M)); (3)the lateral condyle force 306 in the vertical direction (F^(N) _(L));(4) the lateral condyle force 308 in the anterior/posterior direction(F^(T) _(L)); and (5) the force 310 applied by the cam on the post(F_(P)). Point O represents the rotation point of the polyethyleneinsert 312 relative to the tibial implant (not shown) about whichmoments are summated. Also included is the distance r₁ between point Oto the medial condyle contact force, and the distance r₂ between point Oto the lateral condyle.

If the moments are summated for the mobile bearing TKA prosthetic 300,around point O, in the T3> direction (perpendicular to the T1> and T2>directions), the moment equation is represented by Equation #14:ΣM _(o) ·T3>=I·α·T3>  Equation #14:We can assume that the angular acceleration (α) of the polyethyleneinsert 312 relative to the tibial implant component in the T3> directionis negligible, and can be set equal to zero. Therefore, with thispresumption in place, Equation #14 can be refined into Equation #15:ΣM_(o)=0 in the T3>direction.ΣM _(o) ·T3>=−r ₁ ·T2>×F ^(T) _(M) ·T1>+r ₂ ·T2>×F ^(T) _(L)·T1>  Equation #15:Where the following information is known, the distance r₁=r₂=r, and theforces F_(M)=F_(L)=F, Equation #15 can be further simplified intoEquation #16:ΣM _(o) ·T3>=−rF·(−T3>)+rF·(−T3>),ΣM _(o) ·T3>=0.  Equation #16:As shown by Equation #16, if the distances r₁, r₂ from the rotationpoint O of a mobile bearing TKA prosthetic 300 are the same to themedial and lateral condyles, the sum of the moments is equal to zero.Thus, the polyethylene insert 312 does not rotate about the tibialcomponent. An in vivo analysis of the mobile bearing TKA prosthesis 300evidenced that 7/9 subjects experienced less than 2.0 degrees of axialrotation.

Referring to FIG. 4, a typical fixed bearing TKA prosthetic 400 includesa tibial insert 402 (typically polyethylene) mounted to a tibial implantcomponent (typically a metallic tibial tray, which is not shown). Unlikethe mobile bearing posterior stabilized TKA prosthetic 300 of FIG. 3,the tibial insert 402 is fixed to the tibial implant component so thatthe insert does not rotate with respect to the tibial implant component.The forces acting on the fixed bearing posterior stabilized insert 402are similar to those forces defined for the mobile bearing posteriorstabilized TKA prosthetic 300 of FIG. 3.

Summation of moments acting on the fixed bearing polyethylene insert 402can be conducted around the defined point O. The primary differencebetween the point O of FIG. 4, and point O of FIG. 3, is that the chosenpoint O does not represent a rotation point in FIG. 4, but rather afixed physical point on the tibial insert 402 in the center of the postin the T2> direction. Summating the moments around point O isrepresented by Equation #14:ΣM _(o) ·T3>=I·α·T3>  Equation #14:Similar to the mobile bearing TKA prosthetic 300 of FIG. 3, we canassume that the angular acceleration (α) of the femur relative to thetibial insert 402 in the T3> direction is small, and can be set equal tozero. Therefore, with this presumption in place, Equation #14 can berefined into Equation #15:ΣM_(o)=0 in the T3>direction.ΣM _(o) ·T3>=−r ₁ ·T2>×F ^(T) _(M) ·T1>+r ₂ ·T2>×F ^(T) _(L)·T1>,  Equation #15:Where the following information is known, the distance r₁=r₂=r, and theforces F_(M)=F_(L)=F, Equation #15 can be further simplified intoEquation #16:ΣM _(o) ·T3>=−rF·(−T3>)+rF·(−T3>)ΣM _(o) ·T3>=0  Equation #16:

If the polyethylene post 404 is located in the center of the tibialinsert 402, in the T1> and T2> directions, then the sum of the moments,in the T3> direction, for the contact forces applied by the femoralcomponent on the tibial insert is equal to zero. Previous in vivoanalyses of TKA prosthetics have determined that all TKA prostheticsachieve less axial rotation than a natural knee, while a significantnumber of TKA recipients are able to achieve less than two degrees ofaxial rotation of the resulting knee joint and approximately ⅓ of theserecipients experience a reverse axial rotation pattern, opposite that ofa natural knee.

Referencing FIG. 5, a posterior cruciate retaining TKA prosthetic 500(whether mobile or fixed bearing) allows for posterior cruciate ligamentretention, without the presence of a cam/post mechanism 404 found in theposterior stabilized TKA prosthetic 400 of FIG. 4. The absence of thecam/post mechanism in a posterior cruciate retaining TKA prosthetic 500leads to an analysis very similar to those for a posterior stabilizedTKA prosthetics discussed above. Since the cam/post mechanism forpresent day posterior stabilized TKA prosthetics is in the center of thetibial insert, the cam/post force does not exert a moment. Therefore,when the cam/post mechanism does not induce rotation, the momentanalysis for a mobile bearing posterior cruciate TKA prosthetic 500 willbe similar to the moment analysis of a mobile bearing posteriorstabilized TKA prosthetic 300 (see FIG. 3), and the moment analysis fora fixed bearing posterior cruciate retaining TKA prosthetic 500 will besimilar to a fixed bearing posterior stabilized TKA prosthetic 400 (seeFIG. 4), except for the resistive force of the posterior cruciateligament. In other words, the sum of the moments for the contact forcesapplied by the femoral component on the tibial insert is equal to zerofor these posterior cruciate retaining TKA prosthetics 500.

Referencing FIG. 6, an exemplary posterior stabilized mobile bearingMoment Induced Total Knee Arthroplasty (MITKA) prosthetic insert 600 ismounted to a prosthetic tibial stem 606, which is preferably implantedinto a patient's tibia (not shown). The insert 600 includes a post 602offset in the medial direction from the medial-lateral midline 608 ofthe insert 600 providing an axis of rotation 605 between the insert anda femoral component (not shown), and an axis of rotation 604 between thetibial stem 606 and the insert 600 that is shifted in the lateraldirection from the medial-lateral midline 608.

Referring to FIG. 7, a moment analysis of the exemplary MITKA prostheticinsert 600 includes summating the moments, around the point of rotation,O, in the T3> direction. The moment equation is represented by Equation#17:ΣM _(o) ·T3>=−r ₁ ·T2>×F ^(T) _(M) ·T1>−r ₃ ·T2>×F _(P) ·T1>+r ₂ ·T2>×F^(T) _(L) ·T1>  Equation #17:Where the following information is known, the distance r1=2r, r2=r3=r,and the forces F^(T) _(M)=F^(T) _(L)=F_(P)=F, Equation #17 can befurther simplified into Equation #18:ΣM _(o) ·T3>=−2rF·(−T3>)−rF·(−T3>)+rF·(−T3>)ΣM _(o) ·T3>=2rF·T3>+rF·T3>−rF·T3>ΣM _(o) ·T3>=2rF·T3>  Equation #18:In this exemplary moment summation, the moment induced by the exemplaryMITKA prosthetic insert 600 is equal to 2rF, in the clockwise direction(looking down), leading to normal axial rotation of the tibial insert.

Unlike present day mobile bearing posterior stabilized TKA prosthetics300 (see FIG. 3, for example), the tibial insert 600 for the MITKA kneewill rotate in the clockwise direction (looking down) as a result ofcreating a distance between the rotation point O to the post 602 of thetibial insert 600 (distance represented by r₃), increasing the distancefrom the rotation point O to the medial condyle contact force F^(T) _(M)(distance represented by r₁), and decreasing the distance from therotation point O to the lateral condyle contact force F^(T) _(L)(distance represented by r₂), allowing for the lateral condyle to movemore posterior, similar to a natural knee.

The amount of offset created between the post 602 and the rotation pointO of the MITKA prosthetic insert 600 leads to increased axial rotationof the tibial insert relative to the tibial implant component (notshown) in the clockwise direction (T3> direction). An exemplarymathematical model has determined that a 3 mm shift of the post 602 inthe medial direction from the centerline in the medial-lateral directionand a 3 mm shift of the rotation point O in the lateral direction fromthe centerline in the medial-lateral direction leads to 5 to 13 degreesof normal axial rotation, depending on the weight of the patient, thebalancing of the knee, and the amount of force applied by the cam on thepost (see FIG. 8). A second analysis was conducted using the exemplarymathematical model where the post 602 was shifted 6 mm in the medialdirection and the rotation point O was shifted 6 mm in the lateraldirection. The results for this analysis revealed the amount of normalaxial rotation of the polyethylene increased to a range of 10 to 22degrees of normal axial rotation, again, depending on the weight of thepatient, the balancing of the knee, and the amount of force applied bythe cam on the post. A third analysis, where the post 602 was shifted 10mm in the medial direction and the rotation point O was shifted 10 mm inthe lateral direction lead to normal axial rotation of the polyethyleneranging between 20 to 35 degrees, in the clockwise direction. Greatershifts in the medial and lateral direction are also within the scope ofthe disclosure, such as, without limitation, 0.01 to 20 millimeters ofmedial or lateral shift.

Referencing FIG. 9, an exemplary posterior stabilized fixed bearingMoment Induced Total Knee Arthroplasty (MITKA) prosthetic insert 900 inaccordance with an exemplary embodiment includes a cam/post mechanism902 shifted in the lateral direction from the medial-later center 904 ofthe tibial insert. It is to be understood that the corresponding cam ofthe femoral component would be likewise shifted in the lateral directionto accommodate the shifted tibial post.

Referring to FIG. 10, a moment analysis of the exemplary fixed bearingPS MITKA prosthetic 900 insert includes summating the moments, aroundpoint of rotation O, in the T3> direction. The moment equation isrepresented by Equation #19:ρM _(o) ·T3>=−r1·T2>×F ^(T) _(M) ·T1>+r2·T2>×F ^(T) _(L) ·T1>  Equation#19:Where the following information is known, the distance r1=2r, r2=r, andthe forces F^(T) _(M)=F^(T) _(L)=F, Equation #19 can be furthersimplified into Equation #20:ΣM _(o) ·T3>=−2rF·(−T3>)+rF·(−T3>),ΣM _(o) ·T3>=2rF·T3>−rF·T3>ΣM _(o) ·T3>=rF·T3>  Equation #20:In this exemplary moment summation, the moment induced by the exemplaryMITKA prosthetic insert 900 is equal to rF, in the clockwise direction(looking down), leading to normal axial rotation of the femur relativeto the tibial insert.

The amount of offset created between the post 902 and the rotation pointO of the MITKA prosthetic insert 900 leads to increased axial rotationof the tibial insert relative to the tibial implant component (notshown) in the clockwise direction (T3> direction). An exemplarymathematical model has determined that a 3 mm shift of the post 902 inthe lateral direction from the centerline in the medial-lateraldirection leads to a femoral component rotation in the range of 2 to 8degrees, depending on the weight of the patient, the balancing of theknee, and the amount of force applied by the cam on the post. If thepost 902 is shifted 6 mm in the lateral direction, the amount of femoralcomponent rotation increased to a range of 5 to 13 degrees, and if thepost 902 was shifted 10 mm in the lateral direction, the amount of axialrotation again increased to a range of 9 to 25 degrees. Greater shiftsin the medial and lateral direction are also within the scope of thedisclosure, such as, without limitation, 0.01 to 20 millimeters ofmedial or lateral shift.

As discussed previously, the cam/post mechanism can be used in aposterior stabilized TKA prosthetic to generate rotation by creating amoment arm from the rotation point to the post of a mobile bearing TKA,or by shifting the post laterally, increasing the moment arm from thepost to the medial condyle shear force. In the posterior cruciateretaining TKA, moments are primarily induced by offsetting the rotationpoint and building up conformity between the femoral radii and theconcave tibial insert radii.

Referencing to FIG. 11, an exemplary MITKA posterior cruciate retaining(PCR) fixed bearing prosthetic insert 1100 in accordance with thepresent disclosure includes a medial receiver 1104 and a lateralreceiver 1106 that are adapted to receive the medial and lateralcondyles, respectively, of a femoral prosthesis (not shown). In order torotate the tibial insert 1100 clockwise (looking down), the insert 1100includes greater conformity between the medial condyle and the medialreceiver 1104 on the medial side of the tibial insert. In this exemplaryembodiment 1100, the radii of the medial receiver 1104 is greater thanthe radii of the medial condyle, allowing for anterior/posteriortranslation to occur between the medial condyle and the medial receiver.Increased conformity between the medial receiver 1104 and the medialcondyle leads to an increased shear force applied by the medial condyleon the medial receiver of the polyethylene insert, causing a clockwisemoment to occur, especially if the superior surface of lateral side ofthe tibial insert 1100 is either flat or convex in shape, coupled with aflatter shape for the lateral condyle of the femoral prosthesis.

Referencing FIGS. 12 and 13, an exemplary MITKA mobile bearing PCRprosthetic insert 1200 includes the rotation point 1202 (and rotationalaxis) moved in the lateral direction with respect to the center 1204 ofthe tibial implant baseplate (not shown) using the contours between theinsert 1200 and the femoral condyles 1206, 1208. On the lateral side,the lateral femoral condyle 1206 is flatter (similar to the shape of acanoe) and the lateral receiver 1208 of the tibial insert is eithersloped downward in the anterior-to-posterior direction (see FIG. 12( b))or convex (see FIG. 12( c)). On the medial side, the medial femoralcondyle includes greater conformity with the medial receiver 1210 on themedial side of the tibial insert 1200. Therefore, as the shear forcebetween the femoral condyles and the tibial insert increases, the amountof shear force will be greater on the medial side and induce a clockwiserotation of the tibial insert due to: (1) the increased conformity; and(2) the moment arm from the rotation point to the medial shear forcebeing greater than the moment arm from the rotation point to the lateralcondyle force.

One of the main goals for achieving increased weight-bearing flexion fora total knee arthroplasty is the ability to move the lateral condyle inthe posterior direction. In the normal knee, this can be achievedthrough axial rotation or translation of both condyles. Since, in thenormal knee, the medial condyle does not move more than 10 mm in theposterior direction and on average, this amount is less than 5 mm, thelateral condyle achieves posterior contact through femorotibial axialrotation. Moments are introduced in the MITKA so that normal axialrotation could occur and the lateral condyle can achieve greaterposterior contact with increasing knee flexion. This inducement ofmoments is more easily accomplished with a posterior stabilized TKA,where the cam/post force could be used to drive rotation in theclockwise direction or to lever the medial condyle force with respect tothe post. In the posterior cruciate ligament retaining TKA, the abilityto induce moments is more involved. The MITKA posterior cruciateligament retaining knee uses increased conformity between the medialcondyle and the medial receiver of the polyethylene insert. Also, anincreased radius of curvature for the lateral condyle (canoe shaped)allows the lateral condyle contact point to move in the posteriordirection within the first 30 degrees of knee flexion. Therefore, thegoal of achieve posterior contact with the lateral condyle, withincreasing knee flexion can be accomplished in MITKA posterior cruciateligament retaining TKA through the introduction of moments and bychanging the geometrical shapes of the femoral condyles. In this manner,the axis of rotation between the MITKA mobile bearing PCR prostheticinsert 1200 and the femoral prosthetic can be shifted from themedial-lateral centerline of the insert 1200 (and also from theanterior-posterior centerline), while the axis of rotation between theMITKA mobile bearing PCR prosthetic insert 1200 and the tibialprosthetic tray (not shown) can be shifted from the medial-lateralcenterline of the insert 1200 (and also from the anterior-posteriorcenterline of the insert 1200).

Although the increased conformity between the medial condyle and thereceiver in medial aspect of the tibial polyethylene insert and theflatter lateral condyle, contacting either a posterior sloped or convexshaped lateral aspect of the polyethylene insert has been previouslydescribed herein for a posterior cruciate retaining TKA, these designfeatures can be used in any TKA prosthetic type. The above mentioneddesign changes could be used in a PS TKA type to increase axial rotationand could be used in an anterior and posterior cruciate retaining TKAtype to ensure normal axial rotation.

In the exemplary prosthetic inserts of the present disclosure, theamount of medial condyle conformity with respect to the medial receiverof the tibial insert may play a significant role. An additional factorthat may play a significant role in axial rotation, leading to anincrease or decrease in the amount of axial rotation described herein,is condylar balancing at the time of surgery. It is to be understoodthat the mathematical models referenced in the aforementioned discussionincorporated medial and lateral condyle flexion gaps, duringintra-operative ligament balancing, that were equal, leading to themedial condyle contact force being equal to the lateral condyle contactforce. If the medial condyle contact force is greater than the lateralcondyle contact force, the amount of normal axial rotation wouldincrease over those values predicted by the above-referencedmathematical model. In contrast, if the lateral condyle contact force isgreater than the medial condyle contact force, the amount of axialrotation would fall below those values predicted by the model.

Referencing FIG. 14, the exemplary MITKA posterior cruciate retainingTKA prosthetic includes increased conformity between the medial condyleand the medial receiver of the tibial (polyethylene) insert in order toinduce a clockwise moment of the femur relative to the tibia (normalaxial rotation) (see FIGS. 14( a) & (b)). Also, the lateral condyle willachieve greater posterior motion due to the flatter condylar geometry(canoe shaped) at full extension leading a rapid change of the contactposition from full extension to 30 degrees of knee flexion. Thisposterior change of the contact position for the lateral condyle may befurther assisted by the increased posterior slope of the polyethyleneinsert (see FIGS. 14( c) & (d)) or the convex shape of the polyethyleneinsert (see FIGS. 14( e) & (f)).

Referring to FIG. 15, all cam/post mechanisms in present-day TKAprosthetics 1400 include flat surfaces. These flat surfaces lead to thehypothesis that the contact areas would be large, thereby leading toless stress applied by the cam 1402 onto the post 1404. Unfortunately,if rotation of the femoral component (the cam) 1402 occurs with respectto the tibial insert (the post) 1404 in a fixed bearing posteriorstabilized TKA prosthetic, the opposite is true and the contact area1406 between the femoral cam and the tibial post becomes very small. Aprimary concern for decreased contact areas between the flat cam 1402 ona flat post 1404 is edge loading, leading to high stresses that lead topremature tibial insert failure at the post.

Referencing FIGS. 16 and 17, an exemplary MITKA posterior stabilized TKAprosthetic device 1500 in accordance with the present disclosureincludes a tibial insert 1502 with a tibial post 1504 having a roundedposterior surface 1506. This rounded posterior surface 1506 of the post1502 is adapted to interact with a rounded femoral cam 1508 of a femoralprosthetic component 1510. The radius R for the rounded posteriorsurface 1506 of the post 1504 and the rounded cam 1508 are the similar,but the chosen value of R for the MITKA fixed bearing posteriorstabilized prosthetic device 1500 will depend on the amount of rotationdesired. If the MITKA posterior stabilized prosthetic device 1500 isdesigned to incorporate minimal femorotibial axial rotation, then thevalue for R will be higher than the value for R if the MITKA posteriorstabilized prosthetic device 1500 is designed for greater axialrotation.

While the aforementioned exemplary MITKA posterior stabilizedprosthetics have been explained using a tibial component with anintegral post that is adapted to interface with a cam of the femoralprosthetic component, it is also within the scope of the disclosure toincorporate the post into the femoral component and the cam into thetibial insert. Likewise, while the aforementioned exemplary prostheticshave been explained using a tibial component with an integral postadapted to interface with a cavity within the tibial insert, it is alsowithin the scope of the disclosure to incorporate the post into thetibial insert, where the post would be correspondingly received by acavity within the tibial implant.

While the aforementioned exemplary MITKA mobile bearing POR prostheticshave been explained by shifting the post and point of rotation betweenthe insert and tibial component, it should be understood that one mightonly shift the post or only shift the point of rotation to create themoments discussed herein. An exemplary embodiment would include a PORprosthetic device having a post aligned along the medial-lateralmidline, while the point of rotation between the insert and tibialcomponent (tray) would be offset from the medial-lateral midline.Conversely, an exemplary POR prosthetic device may have its point ofrotation aligned along the medial-lateral midline, while the post wouldbe offset from the medial-lateral midline.

It is also within the scope of the disclosure to shift of the contactpoint of the post or point of rotation anteriorly or posteriorly. Mobilebearing prior art prosthetic knee inserts have always had the point ofrotation centered along the medial-lateral midline and along theanterior-posterior midline. By shifting the contact point of the post orpoint of rotation from the prior art centered position, moments areintroduced if the point of contact of the post and point of rotation arenot coaxial.

Those skilled in the art will readily understand that the exemplaryinserts of the instant disclosure are adapted for use in prosthetic kneejoints comprising tibial and femoral components. The plethora of tibialimplants and femoral implants that the exemplary embodiments of theinstant disclosure may be incorporated with, or used in place of, defiesan exhaustive listing.

Following from the above description and exemplary embodiments, itshould be apparent to those of ordinary skill in the art that, while themethods and apparatuses herein described constitute exemplaryembodiments, the invention is not limited to these precise embodimentsand that changes may be made to such embodiments without departing fromthe scope of the invention as defined by the claims. Additionally, it isto be understood that the invention is defined by the claims and it isnot intended that any limitations or elements describing the exemplaryembodiments set forth herein are to be incorporated into theinterpretation of any claim element unless such limitation or element isexplicitly stated. Likewise, it is to be understood that it is notnecessary to meet any or all of the identified advantages or objects ofthe invention disclosed herein in order to fall within the scope of anyclaims, since the invention is defined by the claims and since inherentand/or unforeseen advantages of the present invention may exist eventhough they may not have been explicitly discussed herein.

What is claimed is:
 1. A prosthetic knee implant system comprising: adistal femoral implant component having a medial condyle and a lateralcondyle; a tibial tray; a mobile bearing tibial tray insert rotatablymountable on the tibial tray for articulation with the distal femoralimplant component, the mobile bearing tibial tray insert including amedial condyle receiver spaced apart from a lateral condyle receiver;wherein the mobile bearing tibial tray insert rotates with respect tothe tibial tray around a substantially vertical first rotational axisoffset laterally from a medial-lateral centerline of the mobile bearingtibial tray insert and the distal femoral implant component rotates withrespect to the mobile bearing tibial tray insert about a substantiallyvertical second rotational axis offset medially from the firstrotational axis.
 2. The prosthetic knee implant system of claim 1,wherein the first rotational axis is offset anteriorly from ananterior-posterior centerline of the mobile bearing tibial tray insert.3. The prosthetic knee implant system of claim 1, wherein the firstrotational axis is offset posteriorly from an anterior-posteriorcenterline of the mobile bearing tibial tray insert.
 4. The prostheticknee implant system of claim 1, wherein: the lateral condyle receiverincludes at least one of a convex shape or a sequentially sloped fromanterior to posterior shape; and the medial condyle receiver includes aconcave shape.
 5. The prosthetic knee implant system of claim 1, whereinthe mobile bearing tibial tray insert includes a recess adapted toreceive a projection from the tibial tray allowing the mobile bearingtibial tray insert to rotate with respect to the tibial tray about thefirst rotational axis.
 6. The prosthetic knee implant system of claim 1,wherein the mobile bearing tibial tray insert includes a projectionadapted to be received by a recess within the tibial tray allowing themobile bearing tibial tray insert to rotate with respect to the tibialtray about the first rotational axis.
 7. The prosthetic knee implantsystem of claim 1, wherein there is translational laxity between themedial condyle receiver and the medial condyle during flexion of theknee.
 8. The prosthetic knee implant system of claim 1, wherein themedial condyle receiver and the medial condyle are shaped to have up to12.5 mm of translational laxity at greater than 20 degrees of kneeflexion.
 9. The prosthetic knee implant knee system of claim 1, whereinthe medial condyle is larger than the lateral condyle.